Abstract The Kitaev honeycomb model is an approximate topological quantum error correcting code in the same phase as the toric code, but requiring only a 2-body Hamiltonian. As a frustrated spin model, it is well outside the commuting models of topological quantum codes that are typically studied, but its exact solubility makes it more amenable to analysis of effects arising in this noncommutative setting than a generic topologically ordered Hamiltonian. Here we study quantum error correction in the honeycomb model using both analytic and numerical techniques. We first prove explicit exponential bounds on the approximate degeneracy, local indistinguishability, and correctability of the code space. These bounds are tighter than can be achieved using known general properties of topological phases. Our proofs are specialized to the honeycomb model, but some of the methods may nonetheless be of broader interest. Following this, we numerically study noise caused by thermalization processes in the perturbative regime close to the toric code renormalization group fixed point. The appearance of non-topological excitations in this setting has no significant effect on the error correction properties of the honeycomb model in the regimes we study. Although the behavior of this model is found to be qualitatively similar to that of the standard toric code in most regimes, we find numerical evidence of an interesting effect in the low-temperature, finite-size regime where a preferred lattice direction emerges and anyon diffusion is geometrically constrained. We expect this effect to yield an improvement in the scaling of the lifetime with system size as compared to the standard toric code.
Abstract The Kitaev honeycomb model is an approximate topological quantum error correcting code in the same phase as the toric code, but requiring only a 2-body Hamiltonian. As a frustrated spin model, it is well outside the commuting models of topological quantum codes that are typically studied, but its exact solubility makes it more amenable to analysis of effects arising in this noncommutative setting than a generic topologically ordered Hamiltonian. Here we study quantum error correction in the honeycomb model using both analytic and numerical techniques. We first prove explicit exponential bounds on the approximate degeneracy, local indistinguishability, and correctability of the code space. These bounds are tighter than can be achieved using known general properties of topological phases. Our proofs are specialized to the honeycomb model, but some of the methods may nonetheless be of broader interest. Following this, we numerically study noise caused by thermalization processes in the perturbative regime close to the toric code renormalization group fixed point. The appearance of non-topological excitations in this setting has no significant effect on the error correction properties of the honeycomb model in the regimes we study. Although the behavior of this model is found to be qualitatively similar to that of the standard toric code in most regimes, we find numerical evidence of an interesting effect in the low-temperature, finite-size regime where a preferred lattice direction emerges and anyon diffusion is geometrically constrained. We expect this effect to yield an improvement in the scaling of the lifetime with system size as compared to the standard toric code.
Topological systems are an exciting frontier of condensed matter physics, as well as having deep relationships to quantum computing. Progress continues to be made both towards building practical quantum computers and experimentally identifying topological features in real materials. However, these attempts have to contend with the messiness of the real world including the effects of finite temperatures. In quantum computing thermal errors are studied through the framework of quantum error correction. This allows a characterisation of how damaging these errors are, and has provided many general results on the limits of how well given quantum codes can endure thermal errors. In the first part of this thesis we study the typical response of one of the best known quantum codes (the two-dimensional toric code) when subjected to thermal noise. This typical behaviour is compared to the limiting bounds and we note differences that arise due to entropic effects. Topological features are typically properties of the ground state of a system. In some cases this means finite temperatures can be damaging, washing out the topological properties with excitations above the ground state. But in other cases it has been shown that topological systems can have their own unique finite temperature behaviours, which can be used to identify them. In the second part of the thesis we study the topological model Kitaev's honeycomb. Fixing certain conserved quantities restricts the model to special sectors, some of which carry topological phases. We begin by investigating an intrinsically finite temperature property of these topological phases, thermal edge currents. At too high temperatures, close to the bulk gap, the topological behaviour of these currents breaks down. However, we show that at lower temperatures they can be identified and shown to have the expected properties such as robustness to perturbations. Following this we investigate the thermal states of the full model. Our approach to this allows us to probe the topological order of the model in a way that has not been done before and we identify a number of interesting new features of the finite temperature physics.
We develop and investigate several quantum many-body spin models of use for topological quantum information processing and storage. These models fall into two categories: those that are designed to be more realistic than alternative models with similar phenomenology, and those that are designed to have richer phenomenology than related models. In the first category, we present a procedure to obtain the Hamiltonians of the toric code and Kitaev quantum double models as the perturbative low-energy limits of entirely two-body Hamiltonians. This construction reproduces the target models' behavior using only couplings which are natural in terms of the original Hamiltonians. As an extension of this work, we construct parent Hamiltonians involving only local 2-body interactions for a broad class of Projected Entangled Pair States (PEPS). We define a perturbative Hamiltonian with a finite order low energy effective Hamiltonian that is a gapped, frustration-free parent Hamiltonian for an encoded version of a desired PEPS. For topologically ordered PEPS, the ground space of the low energy effective Hamiltonian is shown to be in the same phase as the desired state to all orders of perturbation theory. We then move on to define models that generalize the phenomenology of several well-known systems. We first define generalized cluster states based on finite group algebras, and investigate properties of these states including their PEPS representations, global symmetries, relationship to the Kitaev quantum double models, and possible applications. Finally, we propose a generalization of the color codes based on finite groups. For non-Abelian groups, the resulting model supports non-Abelian anyonic quasiparticles and topological order. We examine the properties of these models such as their relationship to Kitaev quantum double models, quasiparticle spectrum, and boundary structure.
Alexei Kitaev's toric code is a rich model, that has birthed and stimulated the development of topological quantum computing, error correction, and field theory. It was also the first example of a quantum error correcting code that could resiliently store quantum information in certain types of noisy environments without the need for active error correction. Unfortunately, the toric code loses much of its power as a noise-resilient quantum memory at any finite temperature. Many of the problems with the toric code at finite temperature are likewise shared among its cousin stabilizer codes. These problems can be traced to the proliferation of stringlike ``defects'' in these codes at finite temperature. The aim of this thesis is then twofold.First, I characterize both numerically and theoretically the failure modes of the toric code at finite temperature with fairly modest bath assumptions. I achieve this by numerically sampling the nonequilibrium dynamics of the toric code using a continuous time monte carlo algorithm. From this analysis, I was able to derive an exact expression for the low and high temperature dynamics of the toric code, as well as a regime over which the toric code is ``most'' stable to noise.Secondly, by leveraging the results of this analysis, I construct algorithms that aim to suppress these noise channels. These algorithms are broadly separated into measurement-free and few-measurement protocols. The measurement-free algorithms are reminiscent of dynamical-decoupling pulse sequences, applied in parallel or in serial to a target stabilizer code, and are entirely unitary. I find that measurement-free algorithms can provide a constant factor increase to the lifetime of a large class of stabilizer codes at finite temperature. The few-measurement protocols are more complex, but I provide evidence that they can provide a threshold for the toric code with asymptotically fewer measurements than was previously known to be achievable.
We analyze low energy spectral properties of small toroidal configurations of the Kitaev honeycomb spin model in the Abelian topological phase. We begin with a brief classification of honeycomb lattices on a torus. Then, using the Brillouin–Wigner perturbation theory, we explain the low order finite size effects that can occur in these systems and show how they affect their ground state topological degeneracy. Finally, we demonstrate the accuracy of the perturbative method by means of exact diagonalization, and use the insights into the finite size effects to reconstruct the topological degeneracy in a small example system.
Topological stabilizer codes, such as the toric and surface codes, are leading candidates for fault-tolerant quantum computation. While their decodability under stochastic noise has been extensively studied, the effects of coherent errors, which involve quantum interference, remain less explored. In this work, we study the decodability of toric codes on honeycomb and square lattices subject to X- and Z-type coherent errors generated by the X- and Z-rotations on each qubit. We establish a duality between these decoding problems and 1+1D monitored dynamics of non-interacting Majorana fermions. This duality shows that the Altland-Zirnbauer symmetry class of the dual Majorana dynamics governs the universal structure of the decodability phase diagram. We show that the honeycomb-lattice toric code (hTC) with X-type error is dual to class-DIII dynamics, while the hTC with Z-type error and the square-lattice toric code (sTC) with both error types are dual to class-D dynamics. The key distinction arises from time-reversal symmetry. In class DIII, the generic transition out of the decodable phase is dual to a measurement-induced transition between dynamical phases with area-law and logarithmic entanglement scaling. In contrast, in class D, the generic decodability transition corresponds to a transition between two topologically distinct area-law phases. To explore these transitions in microscopic models, we consider hTC and sTC with X-type errors as representatives and introduce a minimal two-parameter coherent error model with spatially varying rotation angles. Using analytical and numerical methods, we map out the decodability phase diagrams and characterize the universal behavior of the transitions. We find that the decodability of sTC is more vulnerable to spatially varying coherent errors than uniform ones.
In this chapter, we consider Kitaev's honeycomb lattice model (Kitaev, 2006). This is an analytically tractable spin model that gives rise to quasiparticles with Abelian as well as non-Abelian statistics. Some of its properties are similar to the fractional quantum Hall effect, which has been studied experimentally in great detail even though it evades exact analytical treatment (Moore and Read, 1991). Due to its simplicity, the honeycomb lattice model is likely to be the first topological spin model to be realised in the laboratory, e.g., with optical lattice technology (Micheli et al. , 2006). Understanding its properties can facilitate its physical realisation and can provide a useful insight into the mechanisms underlining topological insulators and the fractional quantum Hall effect. The honeycomb lattice model comprises interacting spin-½ particles arranged on the sites of a honeycomb lattice. It is remarkable that such a simple model can support a rich variety of topological behaviours. For certain values of its couplings, Abelian anyons emerge that behave like the toric code anyons. For another coupling regime, non-Abelian anyons emerge that correspond to the Ising anyons. The latter are manifested as vortex-like configurations of the original spin model that can effectively be described by Majorana fermions. These are fermionic fields that are antiparticles of themselves. They were first introduced in the context of high-energy physics (Majorana, 1937) and become increasingly important in the analysis of solid state phenomena (Wilczek, 2009).